This study addresses the question of whether mRNAs for insulin and/or insulin-like peptides are present in the brain and other extrapancreatic tissues in an attempt to understand the role of these peptides in the maintenance of mental health. RNAs are extracted from various tissues using liquid nitrogen pulverization followed by homogenization in the presence of GuSCN. The RNA pellets recovered from CsCl-cushion centrifugation of the homogenate are subjected to oligo-dt columns for the purification of poly A plus minus RNAs which include mRNAs. These isolated mRNAs are electrophoresed on agarose gel followed by blotting to a nitrocellulose membrane. These immobilized mRNAs are then hybridized to a cloned cDNA fragment of proinsulin gene which has been nick-translated with 32P-dCTP. We found that the 32P-cDNA probe is hybridized to mRNA from extrapancreatic tissues under stringent conditions. However, the molecular sizes of these hybridizable mRNA transcripts are different from that detected in the pancreas. Thus, the size of pancreatic mRNA is of 0.5 kilobase, whereas two species of mRNA transcripts detected in the gut, heart and to a lesser extent, liver have approximately 4.2 and 2.2 kilobases. We also detected these two mRNA transcripts in the brain and a cloned cell line NCB-20 (neuroblastoma x fetal hamster brain cell hybrid), suggesting a neuronal location of these transcripts. The nature and function of the proteins translated from these extrapancreatic mRNA is now under investigation.